Hematopoietic cell transplantation (HCT) can cure a variety of benign and malignant hematopoietic disorders, but graft-versus-host disease (GVHD) remains a significant source of transplant-related mortality and morbidity. Donor T cells in the stem cell grafts recognize widely distributed mismatched major (MHC) and/or minor histocompatibility antigens (miHAg), undergo robust expansion and functional differentiation within recipients, and can cause severe damage to host tissues. Classically, Th1 cells are believed to play a critical role in the induction of GVHD; although recently we and others showed that Th17 cells can also cause GVHD. By targeting Th1 and Th17 specific transcription factors (T-bet and ROR?t), we demonstrated that both the Th1 and Th17 subsets contribute to GVHD development, but either lineage alone is sufficient to induce GVHD, and thus both lineages must to be blocked in order to control GVHD. Targeting transcription factors was used to prove the principle, but essentially lacks translational potential. In our current application, e will extend these findings and evaluate clinically applicable approaches to target T cell differentiation. Specifically, we hypothesize that targeting p40, IL-12R1 or their downstream signaling events will disrupt Th1/Th17 differentiation and thus control GVHD. p40 is a common subunit that pairs with p35 or p19 to form IL-12 or IL-23; IL-12R1 is a shared subunit for IL-12 and IL-23 receptors. The rationale to support this hypothesis is that IL-12 signal is essential for Th1 differentiation whereas IL-23 signal is critical for the maintenance of Th17 subset, and either subset can cause acute GVHD. Thus, blocking p40, IL-12R1 or their downstream signaling events may be sufficient to disrupt Th1 and Th17 differentiation leading to GVHD prevention. Specific Aims are: 1) To prevent GVHD by targeting p40; 2) To control GVHD by targeting IL-12R1; and 3) To characterize and target downstream signaling events of IL-12R1. These studies will validate IL-12/23 pathway as a therapeutic target for GVHD prevention at the cytokine, receptor and downstream signaling level. Because clinical grade antibodies for neutralizing p40 or blocking IL-12R1 subunit and small molecular inhibitor for IL-12/23R signaling are currently available, the information learned from these pre-clinical studies can be readily translated into clinical application.